In air cooled heat exchangers, and particularly condensers, heat is transferred from the hot fluid that flows inside the tubes to the ambient air by passive or forced air flow on the external side of the heat exchanger tubes. The heat transfer between a naked tube and the air is very poor. To improve the efficiency of heat transfer, the heat transfer area of the tubes has been increased by adding fins to each tube. However, incompatible heat flow patterns from the fin to the air and from the tube to the fin showed that contact between a fin and tube not always led to efficient heat transfer of the heat in the fluid to the air.
Finned tubes of air cooled condensers are designed either with plate fins that slide onto the tube and are placed at a desired distance from one another on the tube or by continuously wrapped spiral fins.
A major problem in finned tubes that are not made of a single piece is the heat transfer between the tube and the fin.
With respect to one prior art fin design whereby the inner diameter of the plate fins is substantially equal to the outer diameter of the tube, the fins are brazed, galvanized, soldered or welded to the tube. Alternatively, the tube is made of relatively soft material which is inflated by a pressure pump. The tube diameter is thereby increased to facilitate fastening of the fin onto the tube. This fin design is used only on relatively small sized heat exchangers because of the cost.
In spiral wrapped fins that are usually made of a continuous aluminum ribbon, the fin is either embedded by force into a slot that is preformed on the tube or is wrapped into different types of L-shapes, such as wrap-on, knurled or double L.
US 2008/0023180 and US 2010/0155041 disclose finned tubes that are relief structured and that are manufactured by pressing the fin material into grooves formed in the external wall of the tube. The fins, which are substantially parallel to each other, have an annular shape and are substantially perpendicular to the tube.
Even though this fin configuration provides a significant increase in the heat transfer coefficient between the fin and air as a result of the relief structure formed on the fin, the point of contact between a fin and tube is small, constituting a limited and unreliable means of heat transmission between the fin and tube. An additional drawback of such a fin design is that the circumference of the tube surface between adjacent fins is bare, and therefore the metallic tube surface is subject to corrosion when exposed to e.g. precipitation, thereby lowering the heat transfer coefficient due to the build up of corrosion or requiring to be made of expensive corrosion resisting materials.
It is an object of the present invention to provide heat exchanger tubes having a higher tube-fin heat transfer coefficient than those of the prior art.
It is an additional object of the present invention to provide heat exchanger tubes to which fins are connectable by a large area connection that is not labor intensive.
It is an additional object of the present invention to provide a heat exchanger that can made from inexpensive tubes, without risk that they will corrode.
Other objects and advantages of the invention will become apparent as the description proceeds.